The present invention relates to a signal processing apparatus and method and an information storage apparatus which utilize a partial response, and more particularly to a signal processing apparatus, a signal processing method and an information storage apparatus which carry out waveform equalization by utilizing a partial response subjected to a waveform operation of only (1xe2x88x92D) through a recording system for an input code.
In recent years, the recording density of a magnetic disk device has been rapidly increased. The increase is mostly caused by an MR head (a magneto-resistance effect head) having a high sensitivity. At the same time, a signal processing greatly depends on the fact that partial response maximum likelihood detection (PRML) which can be reproduced at a low S/N ratio has been practically utilized based on the conventional peak detection. The partial response maximum likelihood detection removes a waveform interference utilizing the partial response and reduces a noise based on a decrease in a band.
More specifically, the partial response maximum likelihood detection serves to carry out Nyquist equalization by using a recording and reproducing system of the magnetic disk device as a partial response system, and a cosine roll-off characteristic has been known as a transmission characteristic of a Nyquist equalizer and xc2xd of a maximum frequency determined by a code is set to be a Nyquist frequency. A conversion polynomial for a waveform operation of the partial response system is as follows:
U(t)=(1xe2x88x92D)xc3x97(1+D)m 
wherein D represents a delay operator indicative of 1-bit delay and m is a positive integer. A partial response class 4 (PR4) to have m=1 is generally used for the magnetic disk device.
The maximum likelihood detection of the partial response class 4, that is, so-called PR4ML serves to give the following waveform operation to an input code through recording and reproduction for a magnetic disk and to then correct a reproducing error made by a noise or the like through the maximum likelihood detection in accordance with Viterbi algorithm.
U(t)=(1xe2x88x92D)xc3x97(1+D) 
FIG. 1 is a block diagram showing a conventional signal processing apparatus based on the PR4ML. A signal processing apparatus 100 is constituted by a precoder 102, an NRZI recording system 104, a differential detecting section 106, a magnetic reproducing system 108, an equalizer 110, a level detecting circuit 112 and a maximum likelihood detecting circuit 114. For example, an 8/9 RLL code is input as an input code to the precoder 102 to precode 1/(1+D). The NRZI recording system 104 converts an RZ (Return to Zero) code precoded by the precoder 102 into an NRZI (Non-Return to Zero Interleave) code to be recorded on a magnetic disk with a write head through a step-shaped recording current. The differential detecting section 106 reads the magnetic record of the magnetic disk through a read head and detects an impulse waveform to be a differential waveform of the recording signal. The magnetic recording system 108 amplifies a reproduced waveform having the impulse response of the differential detecting section 106. The equalizer 110 carries out Nyquist equalization and equalizes the waveform operation of (1+D) for a reproduced waveform sent from the magnetic reproducing system 108. The level detecting circuit 112 compares the levels of the output waveforms of the equalizer 110 and obtains a level signal having three values of +1, 0 and xe2x88x921. The maximum likelihood detecting circuit 114 carries out the error correction of a level detected signal in accordance with the Viterbi algorithm. FIG. 2 shows a (1+D) transmission characteristic to be used for the PR4ML, the (1+D) transmission characteristic constitutes a low-pass filter and a noise in a high-frequency region is reduced.
In such a conventional PR4ML, however, the 8/9 RLL code is used. Therefore, if a recording density equivalent to that of a 1/7 RLL code for carrying out peak detection is to be obtained, there is a problem in that a recording frequency becomes higher than the recording frequency of the 1/7 RLL code, a bit shift (NLTS) is generated due to the distortion of a recording current waveform and a non-linear distortion to deteriorate an error rate during reproduction and a circuit becomes complicated.
On the other hand, in the peak detection, there is a problem in that the recording frequency can be lower than that of a PRML by the use of the 1/7 RLL code or the like and data are decided based on the presence of a signal peak in a window, resulting in the difficulty of the implementation of the maximum likelihood detection.
Therefore, the present inventors have proposed a (1xe2x88x92D) partial response signal processing method implementing a maximum likelihood detecting circuit without carrying out the (1+D) waveform operation (Japanese Laid-Open Patent Publication No. 10-328446).
On the other hand, in Japanese Laid-Open Patent Publication No. 6-139699 has proposed a magnetic reproducing apparatus for suppressing an unnecessary high-pass noise component to carry out EPR4 equalization in which a signal to noise ratio on a detecting point is improved. Therefore, a reproducing system is constituted by a low-pass filter having the following frequency characteristic,       L    ⁡          (      w      )        =      {                                                      xe2x80x83                        ⁢                          T              ⁢                              {                                                      cos                    ⁡                                          (                      wt                      )                                                        +                  1                                }                                                                                        "LeftBracketingBar"              W              "RightBracketingBar"                         greater than                           π              T                                                                                      0              ⁢                              xe2x80x83                            ⁢                              "LeftBracketingBar"                W                "RightBracketingBar"                                       greater than                           π              T                                                            xe2x80x83                              
(T is a bit cycle)
an equalizing filter having the following frequency characteristic,       E    ⁡          (      w      )        =            1      -              ⅇ                              -            j                    ⁢                      xe2x80x83                    ⁢          wT                            H      ⁡              (        w        )            
(T is a bit cycle)
wherein a frequency characteristic obtained from recording to reproduction is represented by H (w), and a Viterbi decoder of the EPR4ML.
While a recording system carries out the (1xe2x88x92D) waveform operation in the magnetic reproducing apparatus proposed in the Japanese Laid-Open Patent Publication No. 10-328446 by the present inventors, the (1xe2x88x92D) waveform operation is carried out by utilizing a differential characteristic in a reproducing system. Thus, there is a problem in that a circuit structure becomes complicated.
The present invention provides a signal processing apparatus, a signal processing method and an information recording apparatus which implement an improvement in the distortion of a recording current waveform and a reduction in a non-linear shift (NLTS) with a simple structure without increasing a recording frequency for maximum likelihood detection utilizing a partial response.
Moreover, the present invention provides a signal processing apparatus, a signal processing method and an information recording apparatus which implement an improvement in the distortion of a recording current waveform and a reduction in a non-linear shift (NLTS) with a simple structure by lowering a reproducing error rate and enhancing a recording density without increasing a recording frequency for peak detection.
The present invention is characterized by a signal processing apparatus for carrying out a signal processing for a partial response, comprising a code processing unit for performing a (1xe2x88x92D) waveform processing for an input code and an equalizing unit for Nyquist equalizing a code string obtained by the waveform processing in the code processing unit by setting xc2xc of a maximum frequency determined by a code to be a Nyquist frequency fn. Moreover, there is provided a maximum likelihood detecting unit for carrying out maximum likelihood detection for the code string which is Nyquist equalized by the equalizing unit. The maximum likelihood detecting unit carries out the maximum likelihood detection of a code based on three values of xe2x88x921, 0 and +1 of the Nyquist equalization waveform. Furthermore, there is provided a coding unit for converting input data into a code having a lower recording frequency such as a 1/7 RLL code or a 2/7 RLL code to be supplied to the code processing unit.
According to the present invention, thus, the equalized waveform is generated as a narrow-band (1xe2x88x92D) partial response in which the Nyquiest equalization characteristic setting xc2xc of the maximum frequency determined by the input code to be the Nyquist frequency fn is changed to a cosine roll-off characteristic over the whole passband. Consequently, recording is carried out with a recording frequency as low as that of peak detection, a non-linear shift (NLTS) to be a bit shift caused by the distortion of a recording current waveform and a non-linear distortion is eliminated such that the maximum likelihood detection can be carried out. Thus, an error rate during reproduction can be enhanced.
Moreover, a peak detecting section for carrying out the peak detection for the code string which is Nyquist equalized by the equalizing unit may be provided. Also for the peak detection, a narrow-band (1xe2x88x92D) partial response is generated, in which the Nyquist equalization characteristic setting xc2xc of the maximum frequency determined by the code to be the Nyquist frequency is changed to a cosine roll-off characteristic over the whole passband. Consequently, recording is carried out with a recording frequency as low as that of peak detection, a non-linear shift (NLTS)to be a bit shift caused by the distortion of a recording current waveform and a non-linear distortion is eliminated. Thus, an error rate during reproduction due to the peak detection can be enhanced.
Furthermore, the present invention is characterized by a signal processing method of carrying out a signal processing for a partial response system, comprising a code processing step of performing a (1xe2x88x92D) waveform processing for an input code and an equalizing step of Nyquist equalizing a code string obtained by the waveform processing in the code processing section by setting xc2xc of a maximum frequency determined by a code to be a Nyquist frequency fn. Also in this case, moreover, there is provided a maximum likelihood detecting step of carrying out maximum likelihood detection for the code string which is Nyquist equalized at the equalizing step. The maximum likelihood detecting step carries out the maximum likelihood detection of a code based on three values or five values of the Nyquist equalization waveform. Furthermore, there is provided a coding step of converting input data into a code having a lower recording frequency such as a 1/7 RLL code or a 2/7 RLL code to be supplied to the code processing step. Moreover, a peak detecting step of carrying out the peak detection for the code string which is Nyquist equalized at the equalizing step may be provided.
Furthermore, the present invention provides an information storage apparatus for carrying out a signal processing for a partial response system through a recording medium, comprising a recording portion for converting an RZ input code into an NRZI code to be recorded in a recording medium through a write head, thereby giving a (1xe2x88x92D) waveform processing, that is, an operation of
{1/(1xe2x88x92D)}xc2x7(1xe2x88x92D) 
to an RZ input code, and an equalizing section for Nyquist equalizing a reproduction code reproduced from the recording medium through a read head based on an equalization transmission characteristic obtained by dividing, by a transmission characteristic of a magnetic reproducing system, a cosine roll-off characteristic setting xc2xc of a maximum frequency determined by an RZ input code to be a Nyquist frequency fn. Moreover, there is provided a maximum likelihood detecting section for carrying out maximum likelihood detection for the reproducing code string which is Nyquist equalized by the equalizing section. The maximum likelihood detecting section carries out the maximum likelihood detection of a code based on three values or five values of the Nyquist equalization waveform. Furthermore, there is provided a coding section for converting input data into a code having a lower recording frequency such as a 1/7 RLL code or a 2/7 RLL code to be supplied to the recording section. Moreover, a peak detecting section for carrying out the peak detection for the reproduction code string which is Nyquist equalized by the equalizing section may be provided.